A flight plan is the detailed description of the route to be followed by an aircraft within the framework of a planned flight. It comprises notably a chronological sequence of waypoints described by their position, their altitude and their overflight time. The waypoints constitute the reference trajectory to be followed by the aircraft with a view to best complying with its flight plan. This trajectory is a valuable aid both to the ground control personnel and also to the pilot, for anticipating the movements of the airplane and thus ensuring an optimum safety level, notably within the framework of maintaining the separation criteria between aircraft. The flight plan is commonly managed aboard civil airplanes by a system designated by the terminology “Flight Management System”, that will be called the FMS subsequently, which makes the reference trajectory available to the flight personnel and available to the other onboard systems. Essentially with a view to safety, it is therefore necessary to ensure that the airplane follows at least in geographical terms the reference trajectory described in the flight plan, so as notably to maintain the separation distances between aircraft.
With this aim, State organizations and airport authorities have for a very long time been for example obligated in the publication of takeoff and landing procedures. These procedures have for a long time been published in paper form only, according to graphical and textual formalisms. They guarantee the safety of outbound or inbound flights at aerodromes. Subsequently, they are simply designated by the term “published procedures”. But with the advent in avionics of flight management systems like FMS and of navigation and landing units known by the terminology “Global Navigation and Landing Unit” (GNLU), the procedures published in paper form have been found to be unsuitable, or indeed totally outmoded. The necessity to manage in a digital format all the procedures published by State organizations has become apparent.
Currently, the published procedures are provided to various providers of navigation databases by specialized organizations of the states belonging to the International Civil Aviation Organization (ICAO). The textual and graphical formalisms used are defined by the ICAO, but sometimes they are poorly complied with by State organizations. The best known providers are Jeppesen and LIDO. The providers transform the textual descriptions into series of “legs”, to use the terminology by which they are known in the aeronautics business. A “leg” corresponds to a trajectory portion defined by several parameters, such as for example instructions to be followed in terms of position, altitude, heading or course. Subsequently in the present application, the terminology “legs” could be replaced with the terminology “legs”, it being understood that this substitution is of interest only for translation purposes and that an English version of the present application ought preferably to preserve the original term of “leg”. In any event, the term “legleg” must not here be limitative to straight line leglegs, it can also designate curvilinear legs or combinations of straight line legs and curvilinear legs. The series of “legs” or of “legs” are provided in a digital format, the providers being relatively free in their interpretation of the published procedures as series of legs. The databases thus produced by the providers are called navigation databases. An important principle in the production of navigation databases is the non-corruption of the data. This involves ensuring that the digitization method does not alter the published procedure, each series of legs having to be the faithful reflection of a procedure published by a State organization.
Standardization rules are described for civil aviation in a standard known by the acronym ARINC 424. An aim of the ARINC 424 standard is to normalize the process for producing the navigation databases by the various providers, so as to limit the divergences between the navigation databases arising from different providers. The ARINC 424 standard defines notably a set of 23 legs and it also defines all the combinatorics for chaining these legs together, excluding notably certain chains. The legs currently defined in the ARINC 424 standard are enumerated in the table below.
ARINC 424 LegsLegARINC 424 NameMeaningIFInitial FixFixed initial point on the groundCFCourse To a FixJoining/Following of a ground course up to a fixed pointDFDirect to a FixDirect joining (right) of a fixed pointTFTrack between two FixesGreat-circle route between 2 fixed pointsAFArc DME to a FixDefines a circular arc about a specified remote DME beacon, with anaperture limit.RFRadius to a FixDefines a circular arc between 2 fixed points (the 1st point being thefixed point of the previous leg), on a centre of the fixed circle.VIHeading to InterceptDefines a heading to be followed up to interception of the next legCICourse to InterceptDefines a course to be followed up to interception of the next legVAHeading to AltitudeDefines a heading to be followed up to a given altitudeCACourse to AltitudeDefines a course to be followed up to a given altitudeFAFix to AltitudeDefines a course to be followed, starting from a fixed point, up to agiven altitudeVDHeading to DME DistanceDefines a heading to be followed up to interception of a specified DMEarcCDCourse to DME DistanceDefines a course to be followed up to interception of a specified DMEarcVRHeading to RadialDefines a heading to be followed up to interception of a specified radialCRCourse to RadialDefines a course to be followed up to interception of a specified radialFCTrack from Fix to DistanceDefines a course to be followed starting from a fixed point, over aspecified distanceFDTrack from Fix to DME DistanceDefines a course to be followed starting from a fixed point, until itintercepts a DME arc (specified DME distance)VMHeading to ManualDefines a heading without termination (infinite half line)FMFix to ManualDefines a course, starting from a fixed point, without termination (infinitehalf line)HAHippodrome to AltitudeAerodrome circuit, with Altitude exit conditionTerminationHFHippodrome to Fix TerminationAerodrome circuit, with a single lapHMHippodrome to ManualManual aerodrome circuit, without exit conditionTerminationPIFix to ManualSeparation procedure defined by an outbound course starting from afixed point, followed by a U-turn, and interception of the initialseparation course for the return.Thus, the ARINC 424 standard defines 8 types of so-called “fixed” legs, whose commencement or termination is a fixed waypoint on land, published as latitude and longitude. These are the legs of types IF, CF, DF, TF, AF, RF, FC, FD. The ARINC 424 standard also defines 11 types of so-called “floating” legs whose termination consists of the realization of a variable condition, such as for example legs which terminate when the airplane has attained a certain altitude. These are the legs of types VA, CA, FA, VI, CI, VD, CD, VR, CR, VM, FM. The ARINC 424 standard also defines 3 types of so-called “holding procedure” legs which correspond to aerodrome circuits. These are the legs of types HM, HA, HF. Finally, the ARINC 424 standard defines a type of so-called “course reversal” legs which corresponds to an outbound course followed by a return procedure. These are the legs of type PI.
To manage a trajectory using published procedures, current FMS must therefore manage the 23 legs of the ARINC 424 standard enumerated in the above table, as well as implement the chaining together of any pair of legs from among the 23, that is to say be capable of calculating a trajectory chaining the 2 legs together. This combinatorial is also defined in the ARINC 424 standard, which is restrictive in the sense that certain leg chainings are forbidden. By considering only the 2 by 2 combinations, out of 23×23=529 possible combinations, about 360 are permitted in the current ARINC 424 standard. Thus, certain procedures published by State organizations are not embedded in FMS since they cannot be represented on the basis of the ARINC 424 standard.
A technical problem posed relates to the geographical dispersion of the trajectories generated by different systems on the basis of one and the same published procedure. Specifically, the very large number of procedures published by State organizations, the non strict compliance with the directives of the ICAO by State organizations in textually describing the procedures that they publish, the relative freedom of the navigation database providers in interpreting the published procedures, the large number of floating legs used, but also the algorithmic divergences between the systems like FMS calculating the leg chaining trajectories, induce lateral position deviations which may be very sizable between the trajectories generated by different systems on the basis of one and the same published procedure. Two aircraft can therefore theoretically follow the same published procedure, but not follow the same trajectory in practice. This poses a major safety problem.
Another technical problem posed relates to the validation of the systems managing the published procedures. For example, the development and validation of FMS is complex and expensive. Specifically, it is difficult to ensure exhaustive coverage of all derived cases as there are already 360 basic cases. A veritable combinatorial explosion occurs and it becomes complicated to certify that all possible cases have been tested.
To attempt to remedy this, the RTCA (“Radio Technical Commission for Aeronautics”), which takes on notably advisory functions at the international level in the field of air traffic management, has issued a directive aimed at decreasing the geographical dispersion phenomenon. This is directive DO236B. Among other recommendations, directive DO236B advocates the use of a restricted family of legs, namely the 9 legs out of the 23 legs of the ARINC 424 standard which are the least liable to divergent interpretations. These are the 5 fixed legs IF, CF, DF, TF, RF, the floating leg FA, and the 3 holding procedure legs HA, HF and HM. But the directive will only be applied by State organizations, and then by the providers of navigation databases, for about 10% of the published procedures, namely the published procedures that are required to be described with less ambiguity and more precision, for safety reasons or for better management of the airport zone for example. For these new published procedures, the 9 legs of directive DO236B will suffice. As regards the majority of published procedures, they will not be reconsidered by State organizations, because their current precision is sufficient in relation to the local operational context. Database providers will therefore not be able to apply the recommendations of directive DO236B to them, since they are not compatible therewith. For this reason, it will still be necessary to use the 23 ARINC 424 legs. Thus, directive DO236B partially achieves its aim, by virtue of the cooperation of State organizations in the case of the published procedures that they consider at risk. But directive DO236B fails in the case of the more normal published procedures and the phenomenon of geographical dispersion of different systems for one and the same published procedure has therefore not disappeared.